The Carter Thermoquad carburetor is a very versatile unit, but you need a special tool to adjust the preload spring for the secondary air door to unlock its full potential.  This is an improved tool over other designs and has a very precise fit into the lock screw on the TQ carburetor.

I found the original tool (like the Carter 109P-397 or the Miller C-4152) had limitations when you get to a locking nut that has damaged corners.  Those are usually the ones that people have tried to loosen off with a normal screwdriver or one of the cheaper tools and it just slips off, rounding the corners.

This design has a ring that centers the tool in the nut, and has a very positive engagement, even on damaged locking nuts.  This tool WORKS where others do not.

Built from steel and coated for durable long lasting service.  Comes complete with screwdriver.

$70 USD

Thermoquad Air Door Spring Adjusting Tool